I want to check with the hive to make sure we (myself and the shop) are on the right track to solve an overheating issue on my 2014 Prius C. As background, the car has 316,000 miles and prior to this it performed flawlessly. I bought it at 291,000 and the Carfax showed the original owner (who put all the miles on) took meticulous care of it. Being a Prius it should come as no surprise that the car has no engine temp gauge...anywhere. So the only way I know I'm having an issue is when the warning light comes on. The problem only occurs when driving at highway speeds, typically around 70-75mph, after 20-30 minutes. It'll then give the overheat warning. As soon as I back off the gas, the light goes off within seconds. If I get back up to speed, same thing. No leaks, no burning signs...it's all puking out of the overflow. First visit to the shop, they pressure tested the cooling system and found nothing. They checked the block and head gasket to confirm no issues. Nothing. They discovered that the engine fan wasn't kicking on, so that was replaced. Problem solved? Nope. Same symptoms. Again check for leaks, nothing. Changed the thermostat and radiator cap. Problem solved? Nope. Other clue is that when I'm starting from a stop light I can hear sloshing and gurgling at the firewall (heater). This time the shop kept it for days and road tested it several times. They couldn't get it to display symptoms until yesterday. They took it on a long highway drive and had a scanner plugged in. After extended highway running at a good clip, the water temp climbed to 250 and the light came on. Technician backed off the gas and it immediately dropped down to 219 within 30 seconds. They did another check of the block and head gasket and found nothing. They are thinking the water pump isn't spinning fast enough to keep up at extended highway speeds...the Prius works hard at 70+.
Does this sound right to you? Anything else I should talk to them about?